1. Field of the Invention
This invention relates to an information recording medium for recording information by irradiation of a laser beam. More particularly, this invention relates to optical disks capable of recording information and typified by phase change type optical disks such as DVD-RAM and DVD-RW, magneto-optic disks such as MD and MO and write-once type optical disks such as DVD-R.
In this specification, the information recording medium is sometimes referred to as the xe2x80x9cphase change optical diskxe2x80x9d, the xe2x80x9cmagneto-optic diskxe2x80x9d or merely the xe2x80x9coptical diskxe2x80x9d. However, the present invention can be applied to those types of information media that record information by generating heat by irradiating a laser beam and using this heat to cause a change in atomic configuration or a magnetic moment. For this reason, the present invention can be effectively applied to information recording media other than disk-like media, such as optical cards, without being particularly limited to the shapes of the information recording media.
The laser beam, too, will be called sometimes xe2x80x9claser lightxe2x80x9d or merely xe2x80x9clightxe2x80x9d. As described above however, the present invention uses a laser beam capable of generating heat on the information recording medium. Therefore, the effects of the present invention can be obtained so long as the laser beam can provide a multiple interference effect provided by a plurality of interference layers having mutually different refractive indices. Though the present invention is invented by using a red laser (having a wavelength of 645 to 660 nm) but is not particularly dependent on the wavelength of the laser. Therefore, the present invention exhibits its effects for high-density optical disks using laser having a relatively short wavelength such as blue laser and ultraviolet laser.
2. Description of the Related Art
Recently, phase change optical disks such as 2.6 GBDVD-RAM have been put on the market by utilizing their feature of high compatibility with reproduction-only optical disks such as DVD-ROM, DVD-Video, and so forth. However, the 2.6 GBDVD-RAM has not yet satisfied sufficiently the requirements of users in the aspect of its recording capacity, and hopes have been laid on 4.7 GBDVD-RAM and 4.7 GBDVD-RW. Since these phase change optical disks are equivalent to DVD-Video in the recording capacity, they are expected to accomplish optical disks for recording images that replace VTR.
However, many problems remain yet to be solved in order to accomplish the 4.7 GBDVD-RAM. Among others, suppression of cross-erase is a critical problem.
Generally, when a track pitch of an information recording medium is contracted to about 80% of a laser beam spot, leak of reproduction signals from adjacent information recording tracks occurs. Leak of the reproduction signals from the information recorded to the adjacent information recording tracks is referred to as xe2x80x9ccross-talkxe2x80x9d. A land-groove recording system that follows has been developed to solve this problem.
Dimples (groove shapes) are formed on a plastic substrate of a rewritable optical disk for tracking of a laser beam, and information is generally recorded to recesses or protuberances. To improve the recording density (narrower track pitch), however, a system that records the information to both recesses and protuberances has been developed in recent years. The recess and the protuberance of the dimple are called the xe2x80x9cgroovexe2x80x9d and the xe2x80x9clandxe2x80x9d, respectively. Generally, when the track pitch of the information recording medium is narrowed to about 80% of the laser beam spot and the information is recorded to both land and groove, leak of a reproduction signal occurs from an adjacent information recording track (from the groove to the land and vice versa). When the information recorded to the land is reproduced, for example, the reproduction signal from the information recorded to the groove leaks and the information recorded to the land cannot be reproduced accurately.
To solve this problem, JP-A-6-338064 (hereinafter called the xe2x80x9creference 1xe2x80x9d) teaches to set the groove depth to xcex/7 to xcex/5 (where xcex is a laser wavelength) in the land-groove recording system. This system has the feature in that even when the track pitch is narrowed to about 60% of the track pitch, cross-talk (leak from the adjacent information recording track) can be cancelled.
On the other hand, S. Maita et al xe2x80x9cErasable Phase Change Optical Disks for Recording at Low Linear Velocity (II)xe2x80x9d, Proceedings of the 5th Phase Change Recording Research Symposium, pp9-14 (hereinafter called the xe2x80x9creference 2xe2x80x9d) describes a method of improving reflectance. This method disposes a ZnSxe2x80x94SiO2 layer and a SiO2 layer having mutually different refractive indices on an energy beam incidence side of a recording layer and improves reflectance by a multiple interference effect.
The reference 1 does not sufficiently take into consideration a phenomenon (so-called xe2x80x9ccross-erasexe2x80x9d) in which a recording mark recorded to an adjacent information recording track (an adjacent groove in recording to a land or an adjacent land in recording to a groove) is erased. For instance, a distance between a recording layer and a heat sink layer (reflecting layer) is as small as 18 nm in the reference 1. Therefore, it has been found that heat diffuses into an adjacent information recording track through the heat sink layer during recording of information and cross-erase is likely to occur (problem 1).
To solve this problem, Japanese Patent Application No. 10-285008 as a prior application to the present application (but, Applicants do not intend to admit the prior application as the prior art therein.) describes that the distance between the recording layer and the heat sink layer must be greater than at least the groove depth. When the recording layer and the heat sink layer are spaced apart from each other to such an extent that cross-erase can be sufficiently suppressed (at least 65 nm when a laser wavelength is about 645 to about 660 nm), however, another problem develops in that reflectance drops due to the optical interference effect (problem 2).
To improve reflectance, the method described in the reference 2 may be utilized, in principle. However, since the recording layer and ZnSxe2x80x94SiO2 keep mutual contact in this method, the sulfur (S) element in the ZnSxe2x80x94SiO2 layer diffuses into the recording layer and invites the drop of reflectance when recording is repeatedly conducted thousands of times (problem 3).
To avoid the lowering of reflectance, JP-A-10-228676 describes a method that interposes an interface layer formed of a dielectric compound having a high melting point such as SiO2 or Al2O3 between a recording layer and a ZnSxe2x80x94SiO2 dielectric protective layer. Nonetheless, it has been clarified that when a high temperature humidification test is conducted in this method, exfoliation develops between the recording layer and the interface layer (problem 4).
This exfoliation can be suppressed by using a dielectric material of a Crxe2x80x94O type, a Gexe2x80x94N type, etc, in place of the high melting point dielectric compound such as Al2O3, SiO2, etc, described in aforementioned JP-A-10-285008. However, the inventors of the present invention have found out that though these Crxe2x80x94O and Gexe2x80x94N type materials are resistant to exfoliation, they absorb a laser beam and eventually lower reflectance (similar problem to the problem 2). In this case, the problem can be suppressed to a certain extent when the film thickness of the interface layer is reduced. It has been further clarified, however, that when the film thickness of the interface layer is below 5 nm, diffusion of the S atoms from the ZnSxe2x80x94SiO2 dielectric protective layer cannot be suppressed sufficiently (the occurrence of the problem 3).
When the technologies of the references 2 and 3 are combined, thin films of four layers, in all, exist on the laser beam incidence side of the recording layer, and the total number of thin films is undesirably great from the aspect of production. Furthermore, the references 1, 2 and 3 do not sufficiently take cross-erase into consideration. When the track pitch is narrowed, cross-erase develops depending on the film thickness of each layer (similar problem to the problem 1).
It is therefore an object of the present invention to clarify, and to provide, a structure of an information recording medium that satisfies all of suppression of cross-erase (counter-measure to the problem 1), improvement of reflectance (counter-measure to the problem 2), suppression of the lowering of reflectance when over-write is made a large number of times (counter-measure to the problem 3) and suppression of exfoliation defect (counter-measure to the problem 4).
The following information recording media may be used to solve the problems of the prior art technologies described above.
(1) An information recording medium for recording information through a change of atomic configuration and/or a change of an electron state upon irradiation of a laser beam, including at least a substrate having a groove shape of a groove depth dg, a recording layer having a shape reflecting the groove shape and thin films of three layers of a first interference layer, a second interference layer and a first interface layer having mutually different compositions, and disposed in order named from a laser beam incidence side of the recording layer, wherein: thermal conductivity of the first interference layer is smaller than thermal conductivity of the second interference layer, and a refractive index of the second interference layer is smaller than refractive indices of the first interference layer and the recording layer; a first interface layer is interposed between the second interference layer and the recording layer while keeping contact with the recording layer; and a distance between the first interference layer and the recording layer is not greater than the value dg.
Reflectance can be improved when the refractive index of the first interference layer is greater than that of the second interference layer as will be explained in detail in the later-appearing embodiment.
Here, the first interference layer preferably has a greater refractive index than that of a material existing on a laser beam incidence side of the first interference layer while keeping contact with the first interference layer. Generally, the material existing on the laser beam incidence side of the first interference layer is a plastic substrate such as a polycarbonate or an organic material such as a UV-setting resin. The refractive indices of these materials are from about 1.4 to about 1.6. To effectively reflect light between the organic material and the first interference layer, the refractive index of the first interference layer is preferably at least 2.0. A concrete and referred example is a mixture of ZnS and SiO2 because the mixture can accomplish a high refractive index of 2.0 or more, a film formation rate is high, noise is not generated and thermal conductivity is extremely low.
The refractive index of the second interference layer is not greater than 2.0, preferably 1.8 or below. Therefore, the second interference layer preferably contains an oxide having a low refractive index such as SiO2, Al2O3, MgO, or the like, because the refractive index is extremely low.
The inventors of the present invention have found out that the second interference layer formed of such a low refractive index oxide is likely to peel from the recording layer. To suppress this exfoliation, a first interface layer may be sandwiched between the recording layer and the second interference layer. Because the first interface layer exists while keeping contact with the recording film, its melting point is higher than at least the melting point of the recording layer. A preferred material of the first interface layer has high adhesion between the recording layer and the second interference layer.
The present inventors have further found out that an oxide or nitride of a transition metal or a nitride of a semiconductor element such as Ge or Si, that are likely to turn to non-amorphous compounds, has high adhesion, but because they are the non-amorphous compound, free electrons exist in them and light absorption resulting from the free electrons lower reflectance.
However, when the first interference layer, the second interference layer and the first interface layer are combined with one another, the demerit of each layer described above can be offset, and the present invention can obtain an information recording medium that can satisfy all of suppression of cross-erase, improvement of reflectance, suppression of the lowering of reflectance in many-times over-write and exfoliation defect.
As will be explained later in detail in the later-appearing embodiment, the present inventors have found out also that because thermal conductivity of the low refractive index compound such as SiO2, Al2O3, MgO, etc, used for the second interference layer is greater than that of the compound such as ZnSxe2x80x94SiO2 used for the first interference layer, heat diffuses into the adjacent track and cross-erase becomes likely to occur. The present inventors have found out that this problem can be solved when the distance between the first interference layer and the recording layer is set to a value smaller than the groove depth dg.
An optimum value exists for the composition of each layer, as represented by the following information recording medium.
(2) An information recording medium for recording information through a change of atomic configuration and/or a change of an electron state upon irradiation of a laser beam, including at least a substrate having a groove shape of a groove depth dg, a recording layer having a shape reflecting the groove shape and thin films of three layers of a first interference layer, a second interference layer and a first interface layer having mutually different compositions and disposed in order named from a laser beam incidence side of the recording layer; wherein the first interference layer is formed of a mixture of ZnS and SiO2, that has an amount of ZnS within the range of 50 to 95%, and when the sum of the amounts of O, N, S and C in the second interference layer is X, an amount of O is at least 50% of X and the sum of the amounts of Al, Si and Mg is at least 70% of 1xe2x88x92X; the first interface layer is interposed between the second interference layer and the recording layer, keeps contact with the recording layer, and is formed of an oxide or nitride of a transition metal, or a nitride of Si and Ge, or a mixture containing these members; and a distance between the first interference layer and the recording layer is not greater than the value dg.
The construction described above is effective particularly when reflectance drops due to the optical interference effect. In other words, this construction is effective when reflectance is inevitably sacrificed in order to solve the thermal problem such as cross-erase. Speaking more concretely, it is effective for the construction in which a heat sink layer is disposed on the opposite side to the laser beam incidence side of the recording layer and the distance between the heat sink layer and the recording layer is greater than the groove depth dg.
The heat sink layer disposed on the opposite side to the laser beam incidence side of the recording layer is effective for causing heat generated in recording to rapidly escape and for suppressing damage of the recording layer, as will be explained in detail in the later-appearing embodiment. However, the problem occurs in that heat diffusing into the heat sink layer reaches the adjacent track and causes cross-erase. This problem can be solved when a layer having lower thermal conductivity (third interference layer) than that of the heat sink layer is interposed between the recording layer and the heat sink layer and the distance between the recording layer and the heat sink layer is set to a value greater than the groove depth dg, as will be represented later. Nonetheless, reflectance drastically drops in this construction.
Even in such a situation, the constructions of (1) and (2) can suppress the drop of reflectance.
Consequently, a practical low cross-erase medium having high reflectance can be accomplished as will be described below.
(3) An information recording medium described in (1) and (2) described above, which further includes at least one heat sink layer on an opposite side to the laser beam incidence side of the recording layer, and at least one third interference layer between the recording layer and the heat sink layer, and wherein a distance between the recording layer and the heat sink layer is greater than the value dg.
It has been found that an optimum composition exists for the third interference layer as represented in the following paragraph (4).
(4) An information recording medium as described in (3), wherein the third interference layer is formed of a mixture of ZnS and SiO2 having an amount of ZnS of 50 to 95%.
When such a composition is used, the problem occurs in that the S atoms contained in the third interference layer diffuse into the recording layer during many-times over-write and lower reflectance. In such a case, a second interface layer may be interposed between the recording layer and the third interference layer as described in (5). The interface layer material used for the second interface layer is preferably the oxide or nitride of the transition metal or the nitride of the semiconductor elements such as Ge or Si, in the same way as the material of the first interface layer. However, these materials absorb readily light as described above, and they are likely to impede the multiple interference effect and to lower reflectance. In such a case, too, the construction described in (1) and (2) can suppress the lowering of reflectance. Needless to say, it is important in this case that the amount of the S element contained in the second interface layer is smaller than that of the S element contained in the third interference layer as described in (5).
(5) An information recording medium as described in (4), wherein a second interface layer exists between the third interference layer and the recording layer while keeping contact with the recording layer, and the amount of the S element contained in the second interface layer is smaller than that of the S element of the third interference layer.
When land-groove recording is conducted as described in (6), cross-erase becomes a particular problem. In this case, too, an information recording medium having extremely low cross-erase can be accomplished by using the construction described in (3).
(6) An information recording medium described in (3), which conducts information recording both into the groove (groove) and between the grooves (land).
Incidentally, the first interface layer, the second interface layer and the recording layer are generally extremely thin such as several nm. Therefore, there is the case where these layers do not always have a laminar shape but their films exist in spots in the island shape (formation of the thin films in the island shape). However, such spots of the films can be optically neglected as long as the distance between the island-like thin films is about {fraction (1/10)} of the wavelength of the laser beam, and the effects of the present invention are not lost by assuming that a layer having a mean film thickness of the island-like thin film exists. Even when the interface layer such as the second interface layer exists in the island form, for example, the effect of preventing the diffusion of each interference element into the recording layer can be obtained, though not sufficiently. The main object of the first interface layer is to prevent exfoliation occurring between the second interference layer and the recording layer. As to the first interface layer, therefore, there is no problem at all even when it exists in the island shape so long as the material used for the second interference layer does not easily diffuse into the recording layer.